The present invention relates to a device and method for active noise cancellation applicable to medical intra-body sensors.
In many medical procedures, various physiological conditions present within a body cavity need to be monitored. These physiological conditions are typically physical in nature—such as pressure, temperature, rate-of-fluid flow, and provide the physician or medical technician with critical information as to the status of a patient's condition.
One device that is widely used to monitor conditions is the blood pressure sensor. A blood pressure sensor senses the magnitude of a patient's blood pressure, and converts it into a representative electrical signal that is transmitted to the exterior of the patient.
In the prior art, it is known to mount a sensor at a distal portion of a so-called sensor wire and to position the sensor by using the sensor wire in a blood vessel in a living body to detect a physical parameter, such as pressure or temperature. The sensor includes elements that are directly or indirectly sensitive to the parameter.
One known sensor wire has a typical length of 1.5-2 meters, and comprises a hollow tubing running along a major part of the wire and having an outer diameter in the range of 0.25-0.5 mm, typically approximately 0.35 mm. A core wire is arranged within the tubing and extends along the tubing and often extends out from a distal opening of the tubing. The sensor or sensors is/are preferably arranged in connection with the distal portion of the core wire, e.g. at the distal end of the sensor wire.
The present invention is applicable, for example, in relation with a sensor wire of the type described above.
In one application the sensor wire of the type described above is used to measure pressure in blood vessels, and in particular in the coronary vessels of the heart, e.g. to identify constrictions in the coronary vessels. This may be performed by determining the so-called Fractional Flow Reserve related to the vessel. The sensor wire is typically inserted by use of an insertion catheter, which in turn is inserted via the femoral vein or the radial artery, and guided by the inserted catheter to the measurement site.
In order to power the sensor and to communicate signals representing the measured physiological variable to an external physiology monitor, one or more cables or leads, often denoted microcables, for transmitting the signals are connected to the sensor, and are routed along the sensor wire to be passed out from the vessel to the external physiology monitor, via physical cables or wirelessly.
The sensor element further comprises an electrical circuitry, which generally is connected in a Wheatstone bridge-type of arrangement to one or several piezoresistive elements provided on a membrane. As is well known in the art, a certain pressure exerted on the membrane from the surrounding medium will thereby correspond to a certain stretching or deflection of the membrane and thereby to a certain resistance of the piezoresistive elements mounted thereon and, in turn, to a certain output from the sensor element.
In U.S. 2006/0009817 A1, which is incorporated herein in its entirety for the medical devices and methods disclosed within, and which is assigned to the present assignee, an example of such a sensor and guide wire assembly is disclosed. The system comprises a sensor arranged to be disposed in the body, a control unit arranged to be disposed outside the body and a wired connection between the sensor and the control unit, to provide a supply voltage from the control unit to the sensor and to communicate a signal there between. The control unit further has a modulator, for modulating the received sensor signal and a communication interface for wireless communication of the modulated signal.
In U.S. Pat. No. 7,724,148 B2, which is incorporated herein in its entirety for the medical devices and methods disclosed within, and which also is assigned to the present assignee, another example of such pressure measurement system is disclosed. The pressure sensor wire is adapted to be connected, at its proximal end, to a transceiver unit that is adapted to wirelessly communicate via a communication signal with a communication unit arranged in connection with an external device.
In U.S. Pat. No. 6,112,598, which is incorporated herein in its entirety for the medical devices and methods disclosed within, and assigned to the present assignee, and also in U.S. Pat. No. 7,207,227 B2, which is incorporated herein in its entirety for the medical devices and methods disclosed within, further examples of such pressure sensors and guide wire assemblies are disclosed.
In U.S. Pat. No. 7,326,088, which is incorporated herein in its entirety for the medical devices and methods disclosed within, and assigned to the present assignee, a device adapted for reducing leaking current in a guide wire assembly is disclosed. In this known device a guard potential is applied to an insulator or to a conductive guide wire sheath of the guide wire assembly in order to thereby reduce current leakage.
The patient body acts as an electrically conductive volume with a large surface area exposed to surrounding electrical equipment (fluorescent lighting, X-ray power supplies, etc.) as well as a direct conduction path to intra-body electrical devices (pacemakers, neurostimulators, RF ablation devices, ultrasound catheters, etc.). The man-made electrical interference noise created by such equipment thus efficiently couples onto the patient body through capacitive coupling in the case of external equipment and directly through conduction and/or capacitive coupling in the case of intra-body equipment. This is problematic for precision electrical intra-body sensors since interference voltages many thousand times larger than the signals of interest may corrupt the measurements. A simplified illustration of the influence onto the patient body is illustrated in FIG. 1. In FIG. 1, different influencing capacitances are indicated. For example, Cground (e.g., 300 pF) is the capacitance to ground, Cpow (e.g., 3 pF) is the capacitance in relation to the main power supply, and Csup, Ciso are capacitances in relation to a measuring device connected to the body via an intra-body sensor. Furthermore, the main supply Vmain is an alternating (50/60 Hz) voltage of 230 Volt, for example. Rbody is the resistance of the body and may be approximated to 100Ω. This may result in a voltage Vbody across the body of about 2.3 VAC having a frequency of 50 Hz. This voltage Vbody may naturally influence the measurements performed, for example, by an intra-body sensor being a pressure sensor of the kind described above.
Direct electrical shielding of intra-body sensors is not always possible due to potentially life-threatening electrical leakage current situations that may develop due to sensor mechanical failure. Such a mechanical failure is considered a single-mode fault condition (SFC). In accordance with applicable safety standards for medical equipment no SFC event should be allowed to lead to such dangerous situations, i.e. at least two means of protection is required. One applicable safety standard is the IEC 60601 which is a series of technical standards for the safety and effectiveness of medical electrical equipment, published by the International Electrotechnical Commission.
A passive current-limited shielding that traditionally has been used provides the added means of protection for electrical safety essentially comprising an impedance (e.g., a resistor) connected to device ground. This type of shielding is schematically illustrated in FIG. 2 in connection with a sensor guide wire. The shielding is achieved by a passive impedance connected between a shielded outer tubing of the guide wire and device ground. However the shielding provided is very modest, and at low frequencies (<10 kHz) largely ineffective. This makes the sensor measurements highly vulnerable to 50/60 Hz line voltage interference.
FIG. 3 is a schematic simplified circuit of the shielding in FIG. 2 illustrating passive current-limited shielding provided with an impedance circuit having a resistance Rs of 100 kΩ and a capacitance of 3.3 nF resulting in an impedance of approximately 91 kΩ at 50 Hz. The sensor voltage is less than 5 Volt, which results in a leakage current of less than 50 μA in order to fulfil the type “CF” SFC requirement.